1. Field of the Invention
This invention relates to gas sensors. In this specification the term "gas" refers to materials which are normally gaseous, and also to vapours and odours.
There are many circumstances in which it is desired to detect specific gas or gases in an environment; for example, the emission of certain gases from foodstuffs indicating their condition, the presence of certain pollutants in the atmosphere, or the presence of anaesthetic gases in hospital operating rooms and dental surgeries.
2. Summary of the Prior Art
The semiconduction property of polypyrrole has been used for gas detection, but only to a limited extent. Nylander et al (Nylander, C., Armagathi, M. and Lundstrom, I.) Anal. Chem. Symp Series, 1983, 1 203-207 detected ammonia using a sensing element of precipitated pyrrole black on a filter paper substrate. An almost linear DC resistance change response was obtained with high concentrations of ammonia. The device was also sensitive to moisture and to anions. Maisik et al (Maisik, J. J., Hooper, A. and Tofield, B. C.) JCS Faraday Trans. I, 1986, 82, 1117-26 demonstrated a polypyrrole gas sensor using electrochemically prepared films or interdigitated electrodes. They showed that polypyrrole is in fact sensitive to nitrous oxide and hydrogen sulphide. Pelosi and Persaud (Pelosi, P. and Persaud, K. C.) PCT Int. Appln. WO086/01599, 1986 reported on the sensitivity of a number of semiconducting organic polymers to organic vapours and gases. Such polymers included polypyrrole, poly-2-chloroaniline, poly-2-acetonitrile, polyindole and poly-2-isobutylthiazole. The vapours or gases produced changes in DC resistance in various of the polymers. A major problem in utilising the above phenomena is that of obtaining a signal which can be regarded as specific to a particular gas under investigation.